Lean NOx trap/conversion catalyst

ABSTRACT

A process and composition for converting nitrogen oxides present in exhaust gases produced by an internal combustion engine utilize cobalt and at least one alkali metal or alkaline earth metal as a component of an adsorbent. The process involves contacting the exhaust gas with an adsorbent which adsorbs the nitrogen oxides in lean-burn conditions and recurrently reducing the oxygen concentration of the exhaust gas. During such periods of reduced oxygen concentration, the nitrogen oxides are then desorbed and reduced to nitrogen, thereby reducing the concentration of the nitrogen oxides in the exhaust gas. The composition of the adsorbent comprises an oxide support and at least two components loaded on the support and containing cobalt and at least one alkali metal or alkaline earth metal. The composition demonstrates improved activity at higher temperatures and improved thermal stability.

FIELD OF THE INVENTION

[0001] The present invention relates to improvements in emission controlfor partial lean-burn vehicle engines. In particular, the presentinvention is directed to the emission control of nitrogen oxides fromexhaust gas of internal combustion engines, including diesel engines.

BACKGROUND OF THE INVENTION

[0002] Exhaust gases from vehicle engines primarily contain carbonoxides (CO and CO₂), nitrogen oxides (NO_(x)), hydrocarbons, sulfurdioxide, and soot. At present, one of the most significant problems isremoval of nitrogen oxides, NO_(x), which are produced during hightemperature combustion. In the case of “lean-burn” or partial lean-burnengines, in which there is an excess of oxygen in the exhaust gases, thereduction of NO_(x) to N₂ is particularly difficult because reducingcomponents in the exhaust are often completely consumed by the oxygenthat is present in large excess.

[0003] Catalysts are employed in the exhaust systems of automotivevehicles to convert CO, CO₂, hydrocarbons, and NO_(x), produced duringengine operation into more desirable gases. When the engine is operatedin a stoichiometric air/fuel ratio, catalysts containing palladium,platinum or rhodium, also known as “three way catalysts,” are able toefficiently convert all the gases simultaneously. However, when theengine is operated in “lean-burn” conditions, to realize a benefit infuel economy, such three way catalysts are able to convert CO andhydrocarbons, but are not efficient in the reduction of NO_(x).

[0004] Previous attempts to develop a Lean NO_(x) Catalyst (LNC) thatwill selectively catalyze NO_(x) reduction by HC's has met with limitedsuccess. Catalyst materials developed to date that catalyze theHC—NO_(x) reaction allow only about 30 to 50% NO_(x) conversion underconditions of interest. These catalysts are usually either platinum (Pt)group metals (PGM) containing materials that function only at lowtemperatures (150-200° C.) or base metal materials that function athigher temperatures (300-600° C.). The LNC approach on its own, however,is not sufficient to achieve acceptable NO_(x) reduction to allow futurelegislated limits to be achieved.

[0005] Certain or alkali earth metals such as potassium or strontium incombination with platinum are capable of storing or adsorbing nitrogenoxides under lean conditions, or in conditions of excess oxygen. Morespecifically, the platinum first oxidizes NO to NO₂ and the NO₂subsequently forms a nitrate complex with the alkali or alkaline earthmaterial. For simplicity herein, this sequence of reactions andadsorption shall be referred to as nitrogen oxides being adsorbed, eventhough NO is not adsorbed but is actually first converted to NO₂ whichis then adsorbed. In a rich environment caused, for example, by aregeneration pulse, the nitrate is thermodynamically unstable and thestored NO_(x) is released. The NO_(x) then, with the aid of a catalyst,reacts with reducing species in the exhaust gas to form N₂. Theseadsorbents are known as Lean NO_(x) Trap catalysts (LNT).

[0006] Some shortcomings have been identified for the LNT approach.First, a limited operating temperature window exists for the LNT. Aswith three-way catalysts, a minimum temperature is required for NO_(x)adsorption and conversion. However, unlike the three-way catalysts,NO_(x) adsorption and conversion decreases with increasing temperatureabove a certain temperature (usually about 350 to 400° C.), due todecreasing stability of the adsorbed nitrate. A second shortcoming ofthe LNT is the high cost due to the use of platinum group metals.

[0007] There remains a need for improved NO_(x) conversion catalysts forautomotive lean-burn operation emissions.

SUMMARY OF THE INVENTION

[0008] The present invention pertains to a process and composition whichconverts nitrogen oxides present in exhaust gas produced by an internalcombustion engine. The composition of an adsorbent of the presentinvention comprises an oxide support and at least two components loadedon the oxide support and containing cobalt and at least one alkali metalor alkaline earth metal. The most preferred alkali metal is potassium,however other alkali metals may be used, such as cesium. The componentsmay include additional constituents, such as platinum group metals, butin some cases may be used without any additional constituents. Theprocess comprises the steps of contacting the exhaust gas containingnitrogen oxides with an adsorbent of the present invention, where theadsorbent adsorbs the nitrogen oxides in lean-burn conditions, and thenrecurrently reducing the oxygen concentration of the exhaust gas. Thereduction in oxygen concentration is done in a manner to causedesorption of the nitrogen oxides and reduction of the nitrogen oxidesin the exhaust gas to nitrogen.

[0009] In a preferred embodiment of the present invention, the processalso includes contacting the exhaust gases with a catalyst such as aplatinum group metal, before or while contacting the exhaust gases withthe adsorbent. This step may be performed in order to increase theconversion of the nitrogen oxides in the exhaust gas. The selectedcatalyst or the platinum group metal may be deposited on the oxidesupport with the adsorbent, but is preferably disposed on the upstreamside of the oxide support.

[0010] The present invention is also directed to an exhaust system forconverting nitrogen oxides present in exhaust gas provided by an engine.The system includes an upstream catalyst and a downstream catalyst. Theupstream catalyst may be a three way catalyst, a lean NO_(x) catalyst,or an oxidation catalyst. The downstream catalyst is a lean NO_(x) trapof the present invention.

[0011] It is understood that both the foregoing general description andthe following detailed description are exemplary, but not restrictive,of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The FIGURE is a graph showing nitrogen oxide conversion of thecobalt-potassium trap (also referred to herein as an “adsorbent”) alongwith comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention pertains to a process and composition whichconverts nitrogen oxides present in exhaust gas produced by an internalcombustion engine. The phrase converting nitrogen oxides means that atleast some, and preferably almost all, of the nitrogen oxides enteringthe catalyst are converted to nitrogen. The present invention is alsodirected to an internal combustion engine exhaust gas catalyst systemcomprising a nitrogen oxide trap (LNT) (also referred to herein as an“adsorbent”), and the process of converting NO_(x) in exhaust gases fromthe engine.

[0014] The process for converting nitrogen oxides present in exhaustgases produced by an internal combustion engine first comprises the stepof contacting the exhaust gas containing nitrogen oxides with anadsorbent of the present invention capable of adsorbing the nitrogenoxides in lean-burn conditions. The lean burn conditions involve anexcess of oxygen, which is typical in exhaust gases. The adsorbent thenstores the NO_(x) during the conditions of excess oxygen. When theexhaust flowing into the LNT is lean (i.e. high oxygen content), NO inthe exhaust is catalytically oxydized to NO₂ and then stored by theadsorbent as a nitrate salt.

[0015] The second step of the present process comprises recurrentlyreducing the oxygen concentration of the exhaust gas in a manner tocause desorption of the nitrogen oxides and reduction of the nitrogenoxides in the exhaust gas to nitrogen. Reducing the oxygen concentrationin the exhaust gas produces a “rich” environment. Such a richenvironment may be achieved by providing a regeneration pulse, which isa sharp decrease in the air-to-fuel ratio for a short period of time.The term “recurrent” as used herein is meant to encompass bothreductions in the oxygen concentration at regular intervals andreductions at random intervals. The manner of effecting the decrease inoxygen concentration and the extent of the decrease is well-known in theart.

[0016] The stored nitrates during the rich, or oxygen deficient,environment are thermodynamically unstable, and thus are released by theadsorbent. The cobalt of the adsorbent also acts as a catalyst tocatalyze a reaction between the NO_(x) and the reducing species presentin the exhaust gas, to produce N₂. Such a rich air-fuel ratio is usedperiodically as a regeneration pulse to assist in the regeneration ofthe LNT adsorbent. In the oxygen deficient environment present duringthe regeneration pulse, NO_(x) compounds released from the adsorbent arecatalytically reduced to N₂ upon contact with the catalyst in thepresence of carbon monoxide and residual hydrocarbons in the exhaustgas.

[0017] The adsorbent of the present invention comprises cobalt (Co) andan alkali metal or alkaline earth metal, preferably potassium (K), andis loaded on an oxide support. The adsorbent of the present inventionmay be in the form of an LNT. As is well known, the metals referred toherein actually exist within the adsorbent or other catalysts as oxides.In addition, it is believed that an adsorbent of the present inventionincluding cobalt and potassium form simple oxides, as opposed tocompound oxides. The phrase “loaded on” is used to cover all manners inwhich a catalytically or adsorpively active component (e.g., cobalt orpotassium) can be associated with an oxide, such as by being carried onthe oxide or ion-exchanged onto the oxide.

[0018] Any known high surface area oxide support may be used, and thespecifics of the support do not appear to be critical for the presentinvention. Both natural and synthetic zeolites as well as acidic, basicor neutral zeolites may be used as catalyst support material. Further,the support material may also comprise metal oxides such as, but notlimited to, zirconium oxide (ZrO₂) and aluminum oxide (Al₂O₃). Othersupports such as silica, titania, etc. may also be used. Preferredembodiments of the present invention incorporate ceria, or CeO₂, as theoxide support.

[0019] A preferred LNT for the present invention comprises cobalt andpotassium on a ceria support. The Co/alkali and/or alkaline earth metalmixture may be binary or ternary. Binary mixtures include Co/K, Co/Cs,and Co/Ba. Examples of ternary mixtures include Co/K/Cs, Co/K/Ba, andCo/Cs/Ba. The relative amounts or the metals in the components willdepend on a number of factors including the metals used, the exhaustenvironment, and the desired properties of the adsorbent. In many cases,the binary metal oxide particles of Co/alkali metal may contain Co andan alkali metal in mole ratios of between 5:1 to 1:5. More preferably,the binary ratio is between 2:1 to 1:2.

[0020] The LNT of the present invention may additionally include one ormore platinum group metals (PGMs) as part of the catalyst system, inaddition to the cobalt and the alkali and/or alkaline earth metal group.Platinum group metals include platinum, palladium and rhodium or acombination thereof. The amount of PGM to be used is well known in theart, however, a preferred weight percentage range would be from 0.01% to5.0% of the total support material. In the present application, a PGM isnot necessary for conversion of the NO_(x) to N₂; however, it may beadvantageous in some cases. For example, a PGM may be employed toincrease low temperature activity or for higher thermal stability. TheLNT of the present invention, however, does not require the use of a PGMfor the conversion of NO_(x). Cobalt serves as an effective catalyst forthe reaction to take place between the nitrates and the reducing speciesin the exhaust gas to form nitrogen gas.

[0021] The present LNT composition can be used additionally with anupstream catalyst in an exhaust system, where the LNT of the presentinvention is placed downstream from such catalysts. The upstreamcatalyst aids in achieving a greater reduction of nitrogen oxides, atleast during periods of oxygen reduction. Conventionally known three-waycatalysts (TWC), lean NO_(x) catalysts (LNC), or oxidation catalysts maybe used for this purpose. As is well known, these upstream catalysts mayperform other functions, such as HC or CO reduction.

[0022] The composition of the present invention can be made and used ina manner consistent with conventional LNTs. The cobalt and alkali oralkaline earth metal components may be applied to a substrate orwashcoat, which are known in the art, as described in the followingexamples. The catalysts of the present invention, including Co/K aredeposited on the support material, either simultaneously orsequentially. As seen generally from examples which follow, thecomposition can be made by mixing Co or a salt thereof with K or a saltthereof, dissolving the mixture in water, then loading it onto an oxidesupport such as CeO₂. The resulting powder is then dried at temperaturesranging from 60° C. to 150° C., although preferably at 120° C. for 12hours. The dried powder is then calcined at temperatures ranging from400° C. to 600° C., preferably at 500° C. for 4 hours.

[0023] As will be described by the test procedure and examples whichfollow, Co with preferably K or other alkali and/or alkaline earthmetals results in higher NO_(x) conversion, even at high temperaturessuch as 350° C. to 500° C. Thus, an improved LNT catalyst is describedby the present invention. The operating temperature depends on theadsorbent used.

EXAMPLES

[0024] A nitrogen oxide trap according to an embodiment of the presentinvention was prepared as follows. In a slurry impregnation method, asolution of 4.6 g of Co(NO₃)₂.6H₂O and 1.6 g of KNO₃ in 20 ml of waterwas added to a suspension of 10 g of CeO₂ in 50 ml of water. The mixturewas evaporated while being vigorously stirred until achieving a paste,which was dried in an oven for 24 hours at 120° C. The resulting powderwas calcined at 500° C. for 4 hours, then aged at 850° C. for 24 hoursin air.

[0025] The samples were tested in a laboratory powder reactor with aprocedure that consisted of a precondition followed by a lean-richcycled test. 0.1 g of material was loaded in the reactor and the samplewas exposed to a constant total gas flow rate of 200 standard cm³/min.The sample was preconditioned on ramp to 650° C., held for 15 minutes,cooled to 400° C., all under the rich conditions listed Table 1. Thetemperature was held at 400° C. and the cycle of 60 sec. lean/5 sec.rich for 20 cycles. Then the sample was cooled to 150° C. while cycling30 sec. lean/10 sec. rich.

[0026] After preconditioning, the LNT activity was measured at 150° C.by exposing the sample to 10 cycles at 120 sec. lean/5 sec. rich andcalculating NO_(x) conversions for each of the last 3 cycles. Theconversions for the 3 cycles were then averaged and reported as shown inFIG. 1. The LNT activity measurement was then repeated at 150, 225, 350,450, and 500° C. TABLE 1 Gas Compositions GAS RICH LEAN NO 500 ppm 500ppm CO₂  10% 10% O₂ — 12% CO 7.5% — H₂ 2.5% —

[0027] As seen from FIG. 1, the Co/K sample prepared by slurryimpregnation method was compared with Mn/K supported on CeO₂ and Pt/Ksupported on Al₂O₃. The Co/K LNT showed significantly higher percentageconversion of the NO_(x) at higher temperatures than the Mn/K and Pt/K.The following Table 2 compares the results of the Test procedure: TABLE2 NO_(x) Conversion of 850° C. Aged Samples at SV = 25K with 120 sec.lean/5 sec. rich 350° C. 450° C. 500° C.  9% Co/6% K 69% 80% 78% 10%Mn/10% K 12% 33% 30%  1% Pt/6% K 32% 45% 37%

[0028] The present catalyst system shows higher LNT NO_(x) conversionactivity in middle to high temperature ranges (i.e. 350-500° C.)compared to previous adsorbent materials such as Ba, K, Cs, or Mn/K.Additionally, the Co/K material does not require a platinum group metalto convert NO to NO₂ for storage. The metal oxide-supported Mn/K systemshows similar characteristics. As seen from FIG. 1, however, the Co/Ksystem is more active and has a higher thermal durability than the Mn/Ksystem. After aging at 850° C. for 24 hours in air, the Co/K systemshowed substantially better LNT activity than the Mn/K system.

[0029] The following supplemental examples are representative, notlimiting of the invention.

Example 1

[0030] In an incipient wetness impregnation method may also be used, 4.6g of Co(NO₃)₂.6H₂O and 1.6 g of KNO₃ (or 1.6 g of KOAc) were dissolvedin 2.5 ml of water. 10 g of CeO₂ was impregnated with the above solutionvia sequential or co-impregnation. The resulting powder was dried in anoven for 24 hours at 120° C., calcined at 500° C. for 4 hours, then agedat 850° C. for 24 hours in air.

[0031] The NO_(x) conversion of Co/K supported on CeO₂ showed similaractivities at the same composition, regardless of the preparationmethod.

Example 2

[0032] A solution of 9.8 g of Co(NO₃)₂.6H₂O and 3.4 g of KNO₃ in 20 mlof water was added to a suspension of 20 g of CeO₂ in 50 ml of water.The mixture was evaporated while being vigorously stirred untilachieving a paste, which was dried in an oven for 24 h at 120° C. It wascalcined at 500° C. for 4 hours and aged at 850° C. for 24 hours in air.

[0033] NO_(x) conversion (%) at 120 s lean/5 s rich cycles: 76% at 350°C., 81% at 450° C. and 75% at 500° C.

Example 3

[0034] 3.11 g of Co(NO₃)₂.6H₂O was dissolved in 2.3 ml of water, andthen impregnated onto 10 g of CeO₂. The powder was dried in an oven for24 hours at 120° C. and calcined at 500° C. for 4 hours. A 1.6 g of KNO₃was dissolved in 3.5 ml of water, and then impregnated onto the Co/CeO₂.The resulting powder was dried in an oven for 24 hours at 120° C. Thesample was calcined at 500° C. for 4 hours and aged at 850° C. for 24hours in air.

[0035] NO_(x) conversion (%) at 120 s lean/5 s rich cycles: 79% at 350°C., 84% at 450° C. and 76% at 500° C.

Example 4

[0036] 4.69 g of Co(NO₃)₂.6H₂O was dissolved in 2.0 ml of water, andthen impregnated onto 10 g of CeO₂. The powder was dried in an oven for24 hours at 120° C. then calcined at 500° C. for 4 hours. 1.1 g of KNO₃was dissolved in 3.5 ml of water, and then impregnated onto the Co/CeO₂.The resulting powder was dried in an oven for 24 hours at 120° C.,calcined at 500° C. for 4 hours, and aged at 850° C. for 24 hours inair.

[0037] NO_(x) conversion (%) at 120 s lean/5 s rich cycles: 75% at 350°C., 82% at 450° C. and 76% at 500° C.

Example 5

[0038] 3. 11g of Co(NO₃)₂.6H₂O was dissolved in 2.3 ml of water, andthen impregnated onto 10 g of CeO₂. The powder was dried in an oven for24 hours at 120° C. and calcined at 500° C. for 4 hours. 1.1 g of KNO₃was dissolved in 3.5 ml of water, and then impregnated onto Co/CeO₂. Theresulting powder was dried in an oven for 24 hours at 120° C., calcinedat 500° C. for 4 hours and aged at 850° C. for 24 hours in air.

[0039] NO_(x) conversion (%) at 120 s lean/5 s rich cycles: 81% at 350°C., 85% at 450° C. and 79% at 500° C.

Example 6

[0040] A solution of 7.0 g of Co(NO₃)₂.6H₂O and 6.0 g of Ba(NO₃)₂ in 65ml of water was added to a suspension of 20 g of CeO₂ in 50 ml of water.The mixture was evaporated while being vigorously stirred untilachieving a paste, which was dried in an oven for 24 hours at 120° C.The sample was then calcined at 500° C. for 4 hours and aged at 850° C.for 24 hours in air.

[0041] NO_(x) conversion (%) at 120 s lean/5 s rich cycles: 20% at 350°C., 15% at 450° C. and 14% at 500° C.

Example 7

[0042] A 1.38 g of Co(NO₃)₂.6H₂O and 1.4 ml of Cs solution (436 g Cs/L)was dissolved in 3.5 ml of water, and then impregnated onto 10 g ofCeO₂. The powder was dried in an oven for 24 hours at 120° C., calcinedat 500° C. for 4 hours, then aged at 850° C. for 24 hours in air.

[0043] NO_(x) conversion (%) at 120 s lean/5 s rich cycles: 25% at 350°C., 19% at 450° C. and 18% at 500° C.

Example 8

[0044] A solution of 3.1 g of Co(NO₃)₂.6H₂O, 1.1 g of KNO₃ and 3.3 ml ofCs solution (436 g Cs/L) in 20 ml of water was added to a suspension of20 g of CeO₂ in 50 ml of water. The mixture was evaporated while beingvigorously stirred until achieving a paste, which was dried in an ovenfor 24 hours at 120° C., calcined at 500° C. for 4 hours, then aged at850° C. for 24 hours in air.

[0045] NO_(x) conversion (%) at 120 s lean/5 s rich cycles: 25% at 350°C., 48% at 450° C. and 46% at 500° C.

[0046] Example 1 was similar to the test procedure (uses same amount ofCo/K), except an impregnation method was used. Examples 2-5 useddifferent ratios of Co and K as the washcoat. Example 6 used the binarymixture of Co and Ba. Example 7 used Co and Cs. Example 8 used theternary mixture of Co/K and Cs. Resulting NO_(x) conversion of eachexample at varying temperatures is listed in Table 3. TABLE 3 NO_(x)conversion at 120 sec. lean/5 sec. rich cycles LNT 350° C. 450° C. 500°C. Test Co/K on 69% 80% 78% Procedure CeO₂ Example 1 Co/K on 69% 80% 78%CeO₂ Example 2 Co/K on 76% 81% 75% CeO₂ Example 3 K on Co/CeO₂ 79% 84%76% Example 4 K on Co/CeO₂ 75% 82% 76% Example 5 K on Co/CeO₂ 81% 85%79% Example 6 Co/Ba on 20% 15% 14% CeO₂ Example 7 Co/Cs on 25% 19% 18%CeO₂ Example 8 Co/K/Cs on 25% 48% 46% CeO₂

[0047] The results of the Examples for the present invention show arelatively high NO_(x) conversion at the temperature range from 350° C.to 500° C. Example 6-example 8 have used Ba or Cs and have shown lowerNO_(x) conversion than the previous use of K. As stated before, althoughother alkali metals are contemplated for use with the present LNT, thepreferred alkali metal is K. These examples are merely representative ofthe present invention and are not limiting in any way.

[0048] Although illustrated and described herein with reference tocertain specific embodiments and examples, the present invention isnevertheless not intended to be limited to the details shown. Rather,various modifications may be made in the details within the scope andrange of equivalents of the claims and without departing from the spiritof the invention.

What is claimed is:
 1. A composition which adsorbs nitrogen oxidespresent in exhaust gases produced by an engine during lean-burnconditions, said composition comprising: an oxide support; and at leasttwo components loaded on said support and comprising cobalt and at leastone alkali metal or alkaline earth metal.
 2. The composition of claim 1,wherein said at least one alkali or alkaline earth metal is selectedfrom the group consisting of potassium, cesium, and barium.
 3. Thecomposition of claim 1, wherein said at least two components comprisesaid cobalt and said at least one alkali or alkaline earth metal in amole ratio of between 5:1 to 1:5.
 4. The composition of claim 1, whereinsaid at least two components comprise said cobalt and said at least onealkali or alkaline earth metal in a mole ratio of between 2:1 to 1:2. 5.The composition of claim 1, further comprising a catalyst, disposedupstream of said adsorbent, adapted to reduce a portion of said nitrogenoxides, at least during periods of said oxygen reduction.
 6. Thecomposition of claim 5, wherein said catalyst comprises a platinum groupmetal.
 7. The composition of claim 1, where in said oxide supportcomprises ceria.
 8. A composition which adsorbs nitrogen oxides presentin exhaust gases produced by an engine during lean-burn conditions, saidcomposition comprising: an oxide support; and cobalt and potassiumloaded on said support.
 9. A process for converting nitrogen oxidespresent in exhaust gas produced by an engine, said process comprisingthe steps of: contacting said exhaust gas containing nitrogen oxideswith an adsorbent comprising: an oxide support; and at least twocomponents loaded on said support and comprising cobalt and at least onealkali metal or alkaline earth metal, wherein said nitrogen oxides areadsorbed by said adsorbent in lean-burn conditions; and recurrentlyreducing oxygen concentration of said exhaust gas to cause desorption ofsaid nitrogen oxides from said adsorbent and reduction of said nitrogenoxides to nitrogen.
 10. The process of claim 9, wherein said at leastone alkali or alkaline earth metal is selected from the group consistingof potassium, cesium, and barium.
 11. The process of claim 9, whereinsaid at least two components comprise said cobalt and said at least onealkali or alkaline earth metal in a mole ratio of between 5:1 to 1:5.12. The process of claim 9, wherein said at least two componentscomprise said cobalt and said at least one alkali or alkaline earthmetal in a mole ratio of between 2:1 to 1:2.
 13. The process of claim 9,further comprising, prior to said step of contacting said exhaust gascontaining nitrogen oxides with said adsorbent, contacting said exhaustgas with a catalyst adapted to reduce a portion of said nitrogen oxidesat least during periods of said oxygen reduction.
 14. The process ofclaim 13, wherein said catalyst comprises a platinum group metal. 15.The process of claim 9, wherein said oxide support comprises ceria. 16.The process of claim 9, wherein said at least two components furthercomprise a platinum group metal.
 17. The process of claim 9, whereinsaid at least two components further comprise rhodium.
 18. A process forconverting nitrogen oxides present in exhaust gases produced by anengine, said process comprising the steps of: contacting said exhaustgas containing nitrogen oxides with an adsorbent comprising: an oxidesupport; and at least two components loaded on said support andconsisting essentially of cobalt and potassium, wherein said nitrogenoxides are adsorbed by said adsorbent in lean-burn conditions; andrecurrently reducing oxygen concentration of said exhaust gas to causedesorption of said nitrogen oxides from said adsorbent and reduction ofsaid nitrogen oxides to nitrogen.
 19. An exhaust system for convertingnitrogen oxides present in exhaust gas produced by an engine, saidexhaust system comprising: an upstream catalyst capable of reducingnitrogen oxides; and a downstream lean NO_(x) trap compositioncomprising an oxide support and at least two components comprisingcobalt and at least one alkali metal or alkaline earth metal, saidcomponents loaded on said oxide support.
 20. The exhaust system of claim19, wherein said upstream catalyst is selected from the group consistingof a three way catalyst, a lean NO_(x) catalyst, and an oxidationcatalyst.